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An assembly such as an anchor screw, bone plate, offset hook, post,
transverse connector or other spinal anchor for anchoring to bone and
clamping a linkage such as a rod, wire cable or the like. The assembly has
a top member with an open slot to receive the linkage, and a twist-lock
closure cap to close the open end, capturing the linkage in the slot. One
closure cap fits over and around the top member, with a set of segmented
protrusions that extend through a corresponding set of protruding flange
segments spaced about the circumference of the top member. The cap is
configured to rotate and lock against the top member like a flange-locking
bayonet mount when turned through a limited degree of rotation. Slots may
be provided through the face of the closure cap allowing reduction tabs to
project through the cap, and the locking cap may be adapted to a range of
screw, eye, hook, post, connector and other anchor assemblies to securely
clamp the linkage by use of a rotation tool disposed along an access path
aligned with the axis of the screw, thus requiring no lateral clearance
along the longitudinal direction of the linkage element. Locking is
effected by a set screw that passes through the cap and tightens against
the linkage to firmly immobilize it in the anchor assembly. Another cap
embodiment fits into the center of the head with a partial twist locking
motion. Opposed locking surfaces prevent slippage in the radial direction,
and a sloping contact floor surface may apply torque to further increase
pressure on urge mating detent features in an adjacent wall.

1. An anchor assembly for securing a fixation rod, such assembly comprising: a spinal anchor assembly having a screw axis and a head member with a slot and an open distal end
adapted for passage of a linking element therethrough and a cap for closing the open end wherein the head member comprises an upwardly extending body having at least a partial flange portion extending about a perimeter of said head member, the partial
flange portion of the head member extending internally of the perimeter, and said cap fitting within the head member, and wherein the cap includes a cover having a plurality of spaced apart radially extending rim protrusions adapted for passing through
the partial flange portion when the cover is moved along the screw axis and rotating into engagement with the partial flange portion of the head member so as to close the open end and thereby capture the linking member in the slot, said cap further
having a centrally-placed clamping member for tightening down to further clamp the captured linking member, wherein the clamping member is a clamping screw that passes centrally through the cap for locking the linking member in position.

2. An anchor assembly for securing a linkage, wherein the anchor assembly comprises an anchor element configured for attachment to a bone, and having a protruding top member including an open slot for passing the linkage therethrough, and a cap
for closing the open slot of the top member wherein the top member and the cap are adapted to twist-lock together by a partial rotation to cover the slot so as to capture the linkage, and the cap further includes a clamping member for tightening down to
secure the linkage, the top member includes radially inwardly protruding capture protrusions, the cap is a twist-in cap, and the clamping member includes a locking bolt threaded in the cap.

3. The anchor assembly of claim 2, wherein the top member and cap twist-lock together by engagement of respective flange segments on said top member and said cap.

4. The anchor assembly of claim 3, wherein at least some of the flange segments are sloped along a radial direction.

5. The anchor assembly of claim 2, wherein the top member includes a surface forming a circumferential ridge for centering the cap.

6. The anchor assembly of claim 2, wherein the anchor element is an element selected from the group of elements consisting of monoaxial screws, polyaxial screws, hooks, transverse connectors, offset connectors, slotted connectors, wire or cable
anchors, and anchors for the connection of multiple longitudinal members.

7. A spinal anchor assembly for securing a linking element, wherein the anchor assembly includes: an anchor element configured for attachment to bone and having a head with an open slot for receiving the linking element; and a twist-in closure
cap for closing the head to capture the linking element in the slot; said head and said cap each having respective sets of radially-protruding flanges positioned such that the respective sets pass between each other when the flanges are angularly offset
as the cap is moved along an axis of the anchor element, the cap flanges including superior contact surfaces that engage and lock against complementary inferior contact surfaces of the head flanges when the cap is axially rotated such that the cap
flanges angularly align with the head flanges, wherein said radially-protruding flanges have a radially sloping lower surface to prevent disengagement, wherein the respective sets of radially-protruding flanges are configured to urge the head against the
twist-in closure cap as the cap is axially rotated, and wherein the radially-protruding flanges of the head include vertically-oriented ridges, and the radially-protruding flanges of the twist-in closure cap include mating grooves such that the ridges
are urged against the grooves as the twist-in closure cap is axially rotated.

8. The anchor assembly of claim 7, wherein said cap is an internal twist-in cap.

9. The anchor assembly of claim 7, wherein the twist-in closure cap further includes a protruding stop face to prevent over-rotation.

10. A spinal anchor assembly for securing a linking element, wherein the anchor assembly includes: an anchor element configured for attachment to bone and having a head with an open slot for receiving the linking element; and a twist-in closure
cap for closing the head to capture the linking element in the slot; said head and said cap each having respective sets of radially-protruding flanges positioned such that the respective sets pass between each other when the flanges are angularly offset
as the cap is moved along an axis of the anchor element, the cap flanges including superior contact surfaces that engage and lock against complementary inferior contact surfaces of the head flanges when the cap is axially rotated such that the cap
flanges angularly align with the head flanges, wherein the respective sets of radially-protruding flanges are configured to urge the head against the twist-in closure cap as the cap is axially rotated, and further wherein the radially-protruding flanges
of the head include vertically-oriented ridges, and the radially-protruding flanges of the twist-in closure cap include mating grooves such that the ridges are urged against the grooves as the twist-in closure cap is axially rotated.

11. The assembly of claim 10, wherein said radially-protruding flanges have a radially sloping lower surface to prevent disengagement.

12. The assembly of claim 10, wherein said cap is an internal twist-in cap.

14. The assembly of claim 10, wherein the twist-in closure cap further includes a protruding stop face to prevent over-rotation.

15. A spinal anchor assembly for securing a spinal fixation element, comprising: an anchor element adapted for attachment to bone and defining a central longitudinal axis, the anchor element having an open slot for receiving the spinal fixation
element, side walls on opposed sides of the open slot, a proximal portion, a distal portion, and a plurality of anchor flange segments extending from the side walls in a direction toward the central longitudinal axis, the anchor flange segments each
including an inferior contact surface extending in a direction toward the central longitudinal axis and toward the distal portion to define a radial slant; and a closure element for closing the open slot in the anchor element and applying pressure to
the spinal fixation element to capture the spinal fixation element within the open slot, the closure element including a closure body and a plurality of closure flange segments extending from the closure body in a direction that is transverse to the
anchor element central longitudinal axis when the closure element is placed in the open slot, each closure flange segment including a superior contact surface extending in a direction away from the central longitudinal axis and toward the proximal
portion of the anchor element at the radial slant when the closure element is placed in the open slot so that the closure flange segment superior surfaces engage the anchor element flange segment inferior surfaces over a contact area when the closure
element is placed in a closed position in the anchor element open slot; wherein the anchor and closure flanges are configured to permit the closure element to be placed into the open slot in the anchor element in an open position and to be twisted into
the closed position, and the radial slant is configured so that, when the closure element is in its closed position and pressure is applied to secure the spinal fixation element within the open slot in the anchor element, the sidewalls are drawn
together, and further comprising a detent element located on a side wall of the anchor element and a corresponding detent locking element located on the closure element, the detent element and the detent locking element configured to allow the closure
element to be twisted into the closed position and to retain the closure element in the closed position, the retention of the closure element becoming more secure as the sidewalls are drawn together.

16. The assembly of claim 15, wherein the detent element is a longitudinal protuberance and the detent locking element is a longitudinal slot.

Description

BACKGROUND

The present invention relates to orthopedic fastening systems and to mechanisms for securing and locking a linking or stabilizing element, such as a rod, to a bone screw having a slotted head that receives the rod therein. It also relates to
structures or anchor assemblies having such a slotted or open head for receiving a rod, wherein the slotted portion extends from a hook, plate, bracket or positioning arm.

A number of such structures are known. Thus, for example, the widely used Harms T-plate used for stabilizing the cervical vertebrae has a projecting slotted bolt adapted to receive a rod or cable through the slot and clamp down by screwing a nut
along the bolt to bear down against the rod. Several patents show holding structures for a fixation rod that are incorporated in the head of a screw, as in U.S. Pat. No. 5,672,176, or into a small offset plate which itself may be fastened to the bone,
as shown in published International Application WO96/28105. Other systems involve hooks, transverse rod connectors, or tandem connectors. Various tools have been provided for these systems to enable the surgeon to bend and shape the rod to a desired
contour in situ, to position the rod in the slot of a bolt or head, and to secure the rod in position.

Because the rod is the stabilizing member which provides a precise contour, spacing or connection between one or more vertebrae, bones or bone fragments, alignment is quite critical, and the ability to pass the rod through two or more connecting
assemblies requires various actions to form and shape the rod, or align the receiving structures at defined positions or path before final clamping is effected. This may involve positioning and removing the rod several times to check and adjust the
degree of alignment. Thus, it is, generally desirable to have a closure or secure locking mechanism that may be effected with simple installation steps.

One generally accepted locking mechanism simply involves an internally threaded locking nut that may be tightened down along the axis of the screw or slotted shaft, using a tool such as a socket wrench. Another commercial device employs a
bayonet-mount cap that captures or is captured by the screw head, as in U.S. Pat. Nos. 5,346,493 and 5,257,993. Another system utilizes a cap element with a dovetail or dovetail channel that slides over the rod to close the top of the slot and wedge
the rod firmly in position. This latter construction involves no rotation of threaded members, but has the disadvantage that a certain amount of unobstructed lateral space along the rod adjacent to the connection point is necessary for the sliding
installation of the closure cap. Furthermore, the cap inserts or sliding wedge closures, while they eliminate the need for awkward screwing or rotational motion during installation, cannot be used with some existing reduction screws, translation hooks
or other common hardware having lengthy protruding guide members, reduction tabs or the like. Moreover, the wedge/cap closures are a specialized component that may require the user to switch entirely over to a proprietary line of orthopaedic hardware if
he is to utilize the full range of hook, tab, plate and screw fixation points that may be required in spinal surgery.

Accordingly, it would be desirable to provide a closure cap for a bone screw or similar anchor assembly to secure a fixation linkage such as a rod or cable.

It would further be desirable to provide a rod-securing closure mechanism for a screw or anchor that installs simply by a partial turn.

It would further be desirable to provide a rod-securing closure mechanism for a screw or anchor that requires minimal lateral clearance along the rod for its installation.

It would further be desirable to provide a rod securing cap or closure mechanism adaptable to diverse reduction screws.

SUMMARY OF THE INVENTION

One or more of these and other desirable traits are achieved in accordance with the present invention by a fixation assembly wherein a closure cap fits over an opening to close a rod-receiving slot of a fixation screw, hook, post or other anchor
assembly, and capture the rod therein. A set screw threads through the cap and tightens against the rod to further clamp it in the assembly. In one embodiment, the closure cap extends over and around the head of the rod-receiving assembly, which may,
for example include a slotted shaft, post or head, and the cap is adapted to lock together therewith by limited rotation. This may be accomplished in one embodiment construction by arranging the cap to have a set of sector rim protrusions positioned to
fit through a corresponding set of protruding bosses or partial flange segments located on the head, and to rotate into opposition therewith for securing the cap onto the top of the fixation screw. The protrusions or flange segments are angled, along
the radial direction, so that they bear against each other and jam when rotated, thus cannot slip out of engagement. The opposed segments tighten and lock the cap against the head when the cap is rotated through a partial turn of about twenty-three
degrees of arc, like a flange-locking bayonet mount. The cap may have a rim that extends over the outside of the bolt head to engage external flange segments on the head. In a preferred embodiment, the mating portions may located internally in the
head, with flange segments projecting radially inward from the perimeter, and the cap fitting between segments in the head of the bolt and locking with a twist-in motion to capture a rod in the opening. A hold-down set screw threads through a central
opening in the cap and tightens down against the rod to clamp the rod firmly in place. The closing and clamping may each be effected by a driver tool that operates along the axis of the assembly and requires little or no side clearance to rotate either
the cap or the set screw. In the case of the external, twist-on cap, the limited degree of rotation allows the cap to also include slots through the cap to accommodate reduction tabs extending upwardly from the underlying screw or anchor member. The
quick-twist closure cap assemblies of the invention may be adapted to a wide range of screw, hook, eye, plate, connector and other anchor assemblies for rod, cable and other linking elements.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention will be understood more fully from the description below of representative embodiments thereof taken in conjunction with illustrative drawings, wherein

The invention and its range of embodiments will be better understood following a brief description of prior art, illustrating approaches to one- and two-part anchor assemblies, as well as certain common constructions.

FIGS. 1A-1C illustrate prior art anchor mechanisms for securing a fixation rod as used in spinal fixation. FIG. 1A illustrates an anchor screw, while FIG. 1B shows an offset tab having a slotted post for receiving the rod and FIG. 1C shows a
combined system in which anchor screws secure rod-shaped offset elements that, in turn, each terminate in an end that grips a common fixation rod. In the anchor screw of FIG. 1A, a screw 1 with a rounded head 2 carries a slotted top member 3 into which
a rod 5 shown in phantom is clamped by screwing down a threaded press ring or set screw 6. The press ring 6 is turned by engagement along its central portion e.g., by an Allen wrench, and has external threads 6a which fit corresponding internal threads
3a of the top member 3. A separate body 7 fills the space between the inner wall of the top 3 and the ball head 2 of the screw 1, so that when the rod is pressed down by the member 6, the screw head is firmly gripped and all parts are rigidly held
together. An external nut 8 threads over the outside of the top to further strengthen and lock the assembly. For this prior art anchor member, the screw 1, the press member 6 and the nut 8 may all be installed with a straight tool, such as an Allen
wrench or socket wrench, inserted directly along the axis of the screw.

FIG. 1B shows another anchor assembly 15 for receiving a fixation rod 5. In this assembly, an offset tab construction having a body 15a that is anchored by a conventional bone screw 10 and including a slotted post (not numbered) for receiving
the rod, is closed by a cap nut 17 which carries a pressure member 18 centrally thereon to press down against the rod 5 as the nut is tightened. In each of these two constructions, the member 6 or 17 for clamping down against the rod 5 installs by
rotational movement.

Another prior art anchor assembly is illustrated in FIG. 1C. In this article, a slotted body 21 or 22 is carried either on a bone screw (not visible in the FIG.) or on a short length of offset rod 22a. In both cases, the slotted body 21 or 22
receives a rod and clamps it tightly. In this assembly the slotted head member 21 or 22 has angled or dovetailed walls at its upper portion, and a correspondingly shaped sliding cap member 23 is pressed along the dovetail into the upper region, sliding
along the axis of the slot to close the slot and wedge firmly against the rod passing therethrough. As noted above, this construction has a disadvantage that a lateral clearance along the length of the rod is necessary for movement of the closure member
23 into position. Other constructions are shown in U.S. Pat. Nos. 5,346,493, 5,257,993 and elsewhere.

Thus, the art includes both one-piece, and many-piece anchor assemblies, and these may look like screws, or may be specialized elements that are themselves to be anchored by another assembly. As described further below, the present invention
provides a closing and fixing mechanism of enhanced utility, with a structure adaptable to much of this broad range of hooks, screws, connector assemblies and other orthopaedic anchor hardware involving one or more rod, cable, wire or other linking
elements.

FIG. 2 illustrates a first embodiment of an anchor screw assembly 31 of the present invention. As shown, the anchor screw assembly 31 includes a screw 32 and a top member 33 which may be integral with the screw or, like the prior art
construction of FIG. 1A, may be a separate head member that secures to the proximal end of the screw 32. The top member 33 includes a slot indicated generally by 34 for receiving a rod, and contains at its uppermost region 35, a plurality of segmented
or partial flange members 36a, 36b, 36c, 36d which extend radially outward from its perimeter and have respective slots or spaces 37a, 37b therebetween. As further shown in FIG. 2, each of the flange segments 36a, 36b . . . 36d has a lower surface 38,
as best seen in the end views of flange segments 36a and 36c, that engages a closure cap 40 (FIGS. 3A-3C). While not shown, one or more of the flange segments or cap may include a notch, detent or catch or a jamming feature, to prevent rotation in the
opposite sense.

The anchor screw or hook 31 of FIG. 2 is used in conjunction with a closure cap 40 which is shown in an upward-facing view, from below, in FIG. 3C. The cap 40 fits over and around the upper portion 35 of the slotted, rod receiving top member.
As shown FIG. 3, the cap 40 includes a body 41 having a central threaded bore 42 extending therethrough and a pair of dependent side members 43a, 43b on opposed peripheral sides thereof which extend downward on opposed sides surrounding the outer
circumference of the top member 33. Each of the side members 43a, 43b carries mating inwardly directed protrusions 46a, 46b, and 46c, 46d, respectively, which are spaced apart and positioned to correspond to the segmented flange members 36a-36d of the
screw head. In particular, the protrusions 46a to 46d are positioned below the main body of the cap 41 by an amount corresponding to the maximum thickness of the flange segments 36a to 36d, and are rotationally offset so as to pass down through the gaps
between segments and rotate into gripping engagement around the segments by a partial rotation of the cap 40 about the screw head assembly or top member 33, in the manner of a bayonet mounted lid closure. This secures the cap 40 on the top member
closing the slot to prevent movement of the rod or cable from the head along the axial direction of the screw 32. A set screw (not shown) threaded through the aperture 42 is then tightened to clamp firmly down against the rod, cable or other linkage
captured in the slot 34 (FIG. 2).

Advantageously, with the foregoing construction, the anchor screw 31 and the closure assembly 40 as well as the set screw (not illustrated) all install by simple rotational movement of a tool that extends directly along the axis of the screw.
Moreover, as illustrated, the initial locking of the cap on the head assembly is effected by a small rotational movement, substantially less than one-half turn, which corresponds approximately to the length of the lower surface 38 of one flange segment,
or about 20.degree. of rotational movement. Thus a very slight movement is sufficient to capture the rod 5 (FIGS. 1A-1C) within the slot 34 during initial setup or fitting of the fixation rod.

This twist-lock flanged anchoring assembly with a cap structure of the present invention is readily adapted to diverse other fixation screws of known design, and thus in various alternative embodiments and adaptations may carry forward the
advantages of those other designs. Thus, for example, the locking cap assembly of FIGS. 2-3 of the invention may be adapted to an anchor assembly such as a reduction screw, anchor screw, or hook in which the anchor member possesses protruding reduction
tabs that extend upwardly from the head of the anchoring assembly.

Such an embodiment 50 is shown in FIG. 4. In this embodiment, the head 53 of the anchor assembly has a pair of reduction tabs 55 extending upwardly from the sides of the slot. In this case, the invention contemplates a closure cap 60 with a
rim-engaging securing structure similar to that of cap 40 for engagement by a small rotational motion, but the cap structure further includes a pair of arcuate slots 62a, 62b located in its central region and sized for passage of the reduction tabs 55 or
other protruding head structure therethrough. Each of the slots 62a, 62b extends past the edges of the tabs 55, permitting sufficient rotation of the cap to lock the cap in position. The structure of the cap itself strengthens or supports both the
surrounding wall of the rod receiving slot, and the thin-walled tabs 55 which rise therefrom, while leaving the central on-axis region above the cap entirely unobstructed for insertion, for example, of a set screw along an axial direction, and permitting
line-of-sight access by a driver for installation.

In any of the foregoing constructions, the rod-receiving head assembly or top member 33, 53 may be integral with the anchor screw 35, 52 or may be constituted by a separate slotted head member that fits about the top of the screw to grip the rod
or other connecting linkage. Thus, the invention applies to diverse anchors, hooks, monoaxial screws, transverse connections or tandem connections, slotted connectors or the like.

FIG. 4 illustrates this aspect of the invention for a reduction tab embodiment of which the head is separate from the screw. As shown, the anchor screw assembly 50 has a screw body 52 with an enlarged head 54 which may, for example, have an
Allen or other female socket formed therein (not shown) for applying torque along the axis of the screw to insert the screw in bone. A slotted top member 53 having a tapered interior bore is first fitted over the head 54, receiving the screw body from
above, capturing the enlarged ball head 54 of the screw therein. A compressed member (not shown) which may be similar to element 7 of FIG. 1A, may be provided to create a binding fit, and this element may be fixed in place, for example, by swaging at
opposed surface relief drillings 53a, or it may fit by simple compression. The provision of top member 53 as a separate head structure that is loosely fixed to, but originally decoupled from, the screw 52 in this manner allows the slot angle to be set
at a later stage of installation, while avoiding the risk of losing separate small components.

The cap 60 of this embodiment, which is shown in a perspective view from below in FIG. 4A, is similar to that of the first described embodiment, but includes arcuate slots 62a, 62b to accommodate the projecting reduction tabs. In each case, the
cap member having a dependent locking rim that grips the outside of the slotted top and closes the slot by a partial rotation, provides a simple and unobstructed procedure for closing the head of the anchor and capturing the rod, cable or other linkage
in the anchor assembly and clamping the linkage while fixing the orientation.

In each of the foregoing illustrated embodiments, the cap extends radially beyond the outer radius of the anchor screw head assembly, and has a rim that extends to a greater diameter, and slides between the segmented flange bosses 36 to rotate
into a captured position which closes the slot and captures the rod or other linkage within the head of the anchor assembly. A radial slant "RS" may be provided on one or more faces of the opposed locking members as shown in the detail FIG. 4B to assure
that they cannot slip radially outward under pressure.

In further embodiments, the invention contemplates a twist-on cap member which fits within the head of the anchor assembly rather than extending over and locking on the outside of the head.

FIG. 5 illustrates one embodiment 100 of such a twist-in anchor closing mechanism. As shown, the anchor assembly 100 has a screw portion 152 for anchoring in bone, and a head portion 153 for receiving the rod, cable or other linkage. A closure
cap 140 closes the slotted end of head 153. As in the previously described embodiments, the screw and head may be separate assemblies, in which case the upper portion of the screw preferably has a ball end as described above that allows the head to
pivot about the axis of the screw and achieve a further degree of freedom in angular orientation before clamping down. As with the earlier described embodiments, the cap or closure portion 140 may have a central bore 145 which is internally threaded to
accommodate a set screw to further clamp the rod in the slot; however, to simplify the drawing, threads are omitted from FIG. 5.

The internal closure cap 140 has a plurality of radially protruding flange segments 146, of which one is visible in the Figure., and the cap is pushed downwardly on the head so the respective inward and outward directed segments pass between each
other, in a manner similar to the above-described embodiments. Thus, the segments 146 fit between corresponding, inwardly protruding segments 158 of the head 153 and lock thereagainst by a small rotation of the cap 140.

FIGS. 6A and 6B illustrate the structure of the twist-in cap 140 and the slotted head 153 in greater detail.

FIG. 6A shows the closure cap 140, and FIG. 6B shows the head assembly 153, of an internal closure locking cap of FIG. 5. As shown, the head assembly 153 of the anchor screw has a plurality of internally projecting bosses 158 and the closure cap
140 has corresponding outwardly projecting bosses 146. Respective bosses 146, 158 are dimensioned such that the cap 140 may be pushed downwardly between spaces of corresponding bosses to position the upper surface 149 of the cap bosses below the lower
surface 159 of the retaining head bosses 158. As shown in FIGS. 6A and 6B, these mating contact surfaces are angled or sloped downwardly toward the center so that when the cap 140 is rotated to place opposed bosses in an engagement with each other, the
cap exerts a net inwardly directed force on the head to prevent spreading of the retaining slot. The contours of the sloped ends are relatively sharply defined, effectively forming a circumferential ridge 149a, 159a and groove 149b, 159b on each of the
respective components (FIG. 6C). The ridge of one part fits in the groove of the other, so that the closure is centered and grips over a substantial contact area.

As best seen in FIGS. 5 and 6A, the twist-in cap has opposed edge flats 144 which may provide a contact or engagement surface for a tool such as a wrench used for turning the cap upon installation. Each of the flats 144 has a corner to prevent
over-rotation of the cap, so that upon insertion it rotates to exactly position the respective bosses 146, 158 opposite each other as the anchor assembly is closed. The set screw is then tightened to secure the fixation linkage captured in the slot.

FIGS. 7A-7C illustrate another embodiment of a twist-in closure cap and spinal anchor assembly, having an anchor screw, a slotted head and a twist-in closure cap. As in the previously described embodiments, the screw and head may be separate
assemblies, e.g., to achieve freedom in angular orientation before clamping down. The head and cap structure similarly may be adapted to spinal anchor assemblies of other types, such as transverse connectors, anchor plates and other link-receiving
hardware. As with the earlier described embodiments, the cap may have a central bore which is internally threaded to accommodate a clamping set screw to lock and immobilize the linkage once it has been captured in the slot; however, to simplify the
drawing, threads are omitted from FIGS. 7A and 7C.

In this embodiment, the radially protruding bosses or flange segments 146 of the cap, and the inwardly protruding bosses 158 of the head are arranged so the respective inward and outward directed segments pass between each other, when the cap is
pushed downwardly into the head, in a manner similar to the above-described embodiments. Thus, the segments 146 fit between corresponding, inwardly protruding segments 158 of the head and lock thereagainst by a small rotation of the cap. In addition,
the twist-lock mechanism may be configured to exert enhanced contact force in a detent region when the clamp screw is tightened down.

As shown in FIGS. 7A-C, this is achieved in a presently preferred embodiment by providing lower and upper contact faces 159, 149 on the segments 158, 146 of the head and cap, respectively, that slope downward toward the center so that when the
set screw is tightened the upward force on the cap draws the segments 158 inward and upward. A vertically-oriented protruding ridge 158a and mating groove 146a are formed on the head and cap, on or directly above the corresponding flange region of each,
so that the ridge 158a on the head is urged inwardly against the groove surface of the cap. This effectively locks the rotational detent to prevent any rotational movement of the cap once the set screw is tightened.

As further seen in FIGS. 7A and 7C, the twist-in cap has a protruding stop face 164 that contacts the head and prevents over-rotation of the cap when it is turned to close the head. Thus, upon insertion the cap rotates (clockwise as shown) to
position the bosses 146, 158 exactly opposite each other as the anchor assembly is closed. The set screw is then tightened to secure the fixation linkage captured in the slot.

The invention being thus disclosed and illustrative embodiments depicted herein, further variations and modifications of the invention will occur to those skilled in the art. All such variations and modifications are considered to be within the
scope of the invention, as defined by the claims appended hereto and equivalents thereof.